This undated photo provided by the U.S. Court of Appeals for the District of Columbia Circuit shows Judge John G. Roberts, Jr. Roberts is a possible nominee for the U.S. Supreme Court

The confirmation battle in the U.S. Senate over Supreme Court nominee Judge John Roberts will serve as a reminder of the importance of the high court, both within the American system of government and to individual citizens, as well.

Historically, Supreme Court appointments are among the most important a president can make. Since most Supreme Court justices serve lengthy terms on the bench, they often have an impact on law and society long after a president leaves the White House.

President Bush seemed to have that in mind when he nominated 50-year-old federal Judge John Roberts to fill a vacancy on the nine-member court, due to the retirement of Justice Sandra Day O'Connor. "When a president chooses a justice, he is placing in human hands the authority and majesty of the law. The decisions of the Supreme Court affect the life of every American," he said.

The founders of the American republic saw a significant role for the Supreme Court, as part of a complex governmental system of checks and balances.

Congress was established to make laws, while the president's main function is to enforce those laws. It is up to the third branch of government, the judiciary, led by the Supreme Court, to interpret those laws.

Initially there were six justices on the Supreme Court, but that number grew to nine by 1869.

In the early years of the Republic, the Supreme Court exerted relatively little influence. That all changed with an 1803 court decision that established the precedent of judicial review. Since then, it has been up to the Supreme Court to rule whether acts of the president, Congress or state legislatures are done in accordance with the U.S. Constitution.

"Few countries have a situation as strong as ours, where a majority of the Supreme Court can, in interpreting the Constitution -- and there are wide varieties of ways to interpret it -- say that an act of any other branch of government, whether it is the legislature or the president or whomever, states, is unconstitutional. And that ruling becomes the law of the land," said Professor Mary Cheh, an expert on the Supreme Court at George Washington University.

Over its history, the court has settled many major disputes between the president and Congress, and between the federal government and state governments.

The Supreme Court has often reflected the changing nature of American society. In the mid-19th century, the court issued an infamous decision upholding the notion that African-American slaves were not full citizens.

A century later, the high court led the way in forcing states to desegregate their school systems, a ruling that accelerated the struggle for civil rights well into the 1960's.

In 1973, the Supreme Court issued its famous decision legalizing abortion, another example of how its rulings affect the lives of individual Americans.

"So, therefore, the court is the final authority, as it happens in our scheme, since most things become legal questions, the final authority on probably all of the important issues in the country. So, few countries have it quite that stringent and quite that powerful a role for their supreme court," said Professor Cheh.

In recent years, the court has often wrestled with the power of the federal government to regulate various segments of the private sector.

Allan Lichtman is a presidential historian at the American University in Washington. "At stake in judicial appointments, of course, are abortion rights. But it goes far beyond that. Also at stake are issues regarding the separation of church and state, interpretation of the environmental laws, interpretation of product liability laws. All of these things are critically at stake in the judiciary, which serves for life."

Supreme Court justices are appointed by the president, and must be confirmed by the Senate.

Opposition Democrats, like Senator Ted Kennedy of Massachusetts, say the confirmation hearings for Judge John Roberts should give Americans some insight into his judicial philosophy. "I look forward to those hearings, and I think the American people ought to pay a great deal of attention. This is going to have an enormous influence and impact in their lives," he said.

The Supreme Court is different from the other two branches of government, in that its members can serve for life, and are not subject to recall by the voters, as are presidents and members of Congress.

But the high court often reflects the political divisions in the country at any given time. In recent years, the Supreme Court has issued several 5-4 decisions on a host of important legal issues, including the death penalty, the rights of criminals and the viability of what is known as affirmative action, hiring preferences for women and minorities.

"The problem is that we have a country that is divided right down the middle, as we saw in the last election. President Bush was elected by a fairly small margin for a (sitting) president during times of war. When you have that type of political environment, it is inevitable that fights for the Supreme Court are going to be difficult. The Supreme Court itself, ironically, reflects the country fairly well. It is divided right down the middle," said Jonathan Turley, a constitutional law expert at George Washington University Law School.

If confirmed by the Senate, Judge Roberts would replace retiring Justice Sandra Day O'Connor, who has often been a pivotal vote on a divided court.

The Supreme Court is also the most secretive of the three branches of government. Oral arguments before the high court are open to the press and public, but the justices debate and decide the cases in private, often through written briefs, and rarely answer media questions about how or why they decided a specific case.